Phosphate is a very important nutrient for the photosynthesis of plants so that more than 90% of the global production of phosphate is processed to phosphorus fertilizers. The availability of phosphorus, however, is limited. In order to enhance the limited availability of phosphoric material there have been efforts to use sewage sludge, which often contains a considerable amount of phosphorus and which usually is disposed of in a landfill, as a source to recover phosphorus. It has been proposed to burn phosphorus containing sewage sludge to obtain a phosphorus containing ash with a phosphorus content of 8 to 20 wt.-%. The main chemical components of the resulting sewage sludge ashes are SiO2, CaO, Al2O3, Fe2O3 and P2O5. The ashes, however, also contain heavy metals, such as lead, cadmium, chrome, copper, nickel, zinc or mercury. As the content of these elements exceed the limits provided by official regulations, the sewage sludge ash as such cannot be used as fertilizer and has to be processed beforehand.
Document DE 10 2004 059 935 A1 describes a process for the separation of heavy metals from phosphorus-containing sewage sludge ash, wherein the phosphoric ash is heated to a temperature above the boiling point of the oxychlorides or chlorides of the heavy metals to be removed and wherein the heated sewage sludge ash is contacted with an oxygen-lean atmosphere containing chlorine gas. Thereby chlorides and oxychlorides of the heavy metals are formed and evaporated so that they can be separated from the ash. The process has the disadvantage that the hot combustion gases and the gaseous reaction products form highly corrosive gas mixture with the chlorine gas so that there presently is no technology for recovering the heat or keeping the gas stream in a closed loop at temperatures above 850° C. Without heat recovery, the whole volume of combustion air and chlorine gas must be heated from ambient temperature to a temperature above the boiling points of the chlorides and cooled after the reactor by direct supply of fresh air or water. Subsequently, the excess chlorine gas must be removed from the flue gas and disposed of at high costs. New chlorine gas must be produced and supplied by an energy intensive process. As excess chlorine must be provided to ensure a sufficient removal of the heavy metals, the process gas volume is increased considerably.